The metal oxide semiconductor field effect transistors (MOSFETs) are the main devices used in the integrated circuits, and especially in the very large scale integrated circuits. As the MOSFETs are scaled down, Aluminum interconnection has been replaced by copper interconnection during the MOSFETs manufacturing processes, and the MOSFET gate material has been developed from tungsten compound, titanium compound and cobalt compound to nickel compound, and even to metal (Al2O3, Ta2O5, etc.). In addition, by the new technology, the advent of the application of ruthenium as the electroplating seed layer and manganese as the copper barrier layer has diversify the types of the metals that can be used during the fabrication of the MOSFETs, which provides more choice in the MOSFETs device manufacturing.
However, if these metals are attached to the backside of a wafer during the processes, the subsequent processing devices may be contaminated, and then another wafer entering into the subsequent processing devices may also be contaminated, thereby causing the cross contamination between the silicon wafers and the processing devices. And more especially, since some manufacturing processes are required to be performed under a fairly high temperature (even higher than 1000° C.), these metals may even diffuse into the internal of the wafer, thereby leading to a failure of the entire device. Consequently, it is very important and indispensable to control the metal contamination at the backside of a wafer during every manufacturing process step of a MOSFET device, as well as to reduce the contaminant levels in the processing devices.
Chemical vapor deposition (CVD) is one of main process in the MOSFET device fabrication. It is well-known that before the CVD process for forming a thin film on the wafer, the CVD chamber should be cleaned to remove the accumulated coatings on the interior chamber surfaces and the wafer heater, as well as to remove the suspended particles in the chamber, so as to reduce the contaminant levels in the CVD chamber. In the cleaning process, a cleaning gas comprising NF3 is introduced in the chamber, and excited into fluorine plasma through ionization, the fluoride plasma reacts with the coatings on the surfaces of the interior chamber and the wafer heater to generate a fluorine-containing gas, which is exhausted by a pump afterwards, thus to achieve the purpose of cleaning the chamber. Once the chamber has been sufficiently cleaned by the cleaning gas and the cleaning by-products have been exhausted out of the chamber, a season step is performed to seal the remaining contaminants in the processing region of the chamber and further reduce the contamination level, thus to make the atmosphere in the chamber more suitable for the CVD process. This season step is typically carried out by depositing a season film to coat the surfaces of the interior chamber and the heater. When depositing an amorphous carbon film (APF) using the processing device commercially available from Applied Materials, Inc., the applicant finds that the content of metal aluminum at the backside of the wafer seriously exceeding the industry standard (aluminum<1 e11 atom/cm2, other metals<5 e10 atom/cm2) by total reflection X-ray fluorescence (TXRF). Through experiments, it is found that in the CVD chamber provided by the Applied Materials, a very thin layer of AlxFyOz is formed on the surface of the heater through the reaction between the fluoride ions generated from the cleaning gas NF3 and the AlN material of the heater, and then the AlxFyOz further reacts with the N2 gas, which is introduced in the chamber after the cleaning process with C2H2 gas for APF film deposition, to generate the AlN, thereby making the surface of the APF film containing AlN and causing Aluminum contamination more than 1 e11 atom/cm2 at the backside of the wafer when contacting with the APF film.
Therefore, there exists a need for a method for further reducing metal contaminants at the backside of a wafer within a CVD chamber.